The present invention relates to the field of producing substrates comprising at least one thin layer formed on the surface of a support, such substrates being utilised in the fields of microelectronics, nanoelectronics or microtechnology, nanotechnology in a general sense.
The present invention has particularly advantageous applications in the field of materials having electronic, optoelectronic, supra-conductor or piezoelectric functions for example.
For instance, certain electronic and optoelectronic applications can necessitate the use of ternary or quaternary semiconductor materials. However, the number of these ternary and quaternary materials of high structural quality possible to obtain by epitaxial growth is limited as it is rarely possible to find a substrate whereof the crystalline network is adapted to that of the semiconductor layer to be grown. Consequently, heteroepitaxy carried out in lattice conflict causes formation of a significant quantity of structural defects beyond a critical thickness, which then causes irreversibly undesirable modifications of the expected physical properties of the epitaxied layers. Furthermore, the use of strained compound or simple semiconductor layers can be for profiting from the improvement of certain properties. There again, the use of a technique for deforming the layers homogeneously would be an advantage.
To try to eliminate these problems, growth techniques have been developed including producing buffer layers whereof the objective is to absorb the strains induced by the difference in the lattice parameters between the substrate and the epitaxied thin layer.
A first family of techniques aims at using compliant substrates consists of producing epitaxy in conflict with lattice on a fine membrane, serving as a nucleus layer. The strain energy is thus supposed to be relaxed elastically or plastically by the membrane.
By way of example a technique of elastic compliance of a nucleus membrane has been described by S. I. Romanov et al., Appl. Phys. Lett. 75, (1999) p. 4118.
This technique consists of:                porosifying the surface of a substrate of Si so as to form two porous layers with specific upper surface, with the surface layer presenting low porosity while the latter which is inserted between the surface layer and the substrate exhibits increased porosity,        lightly oxidising (maximum monolayer) the surface of the resulting substrate so as to mechanically stabilise the crystallites,        deoxidise the substrate in the growth structure just prior to epitaxy of a fine layer of Si, and        producing growth in conflict with lattice of a layer of SiGe on the fine layer of Si.        
The process described by Romanov et al thus comprises generating epitaxial growth in conflict with lattice on a membrane of Si obtained on porous Si. An effect of compliance (deformation) of the porous layers seems to be observed.
Different studies on the compliant substrate have been carried out. The article by A. M. Jones, Appl. Phys. Lett. 74, (1999) p. 1000 can be cited by way of example, describing a growth technique on a free membrane aimed at depositing on a substrate two layers, namely a first layer which is called sacrificial and then the fine membrane on which epitaxy is performed. The sacrificial layer is a layer whereof the chemical attack speed for a solution is considerable before that of the membrane. A lithographic step is taken to have only one disc of the membrane subsist at the surface of the sample which is then dipped in the chemical attack solution. The sacrificial layer is etched, including under the disc, by sub-etching. The chemical attack is stopped when there is only one pillar remaining to carry the disc of the membrane on which is formed the growth of an epitaxied layer. The membrane is thus capable of deforming to limit the deformations of the epitaxied layer. The advantage of this method is that the epitaxied layer strain is relatively well relaxed elastically. However, this method has a number of disadvantages such as fragility of the structure, non-planarity of the surfaces, difficulty of the production process and the small size of the zones obtained.
A method of misaligned fusion or << twist bonding >> can also cited, as described especially in the article by Y. H. Lo, Appl. Phys. Lett. 59, (1991) p. 2311, transferring a membrane strain or not, on a substrate host by ensuring the generation of a rotation between the crystallographic directions of the membrane and that of the substrate. This creates a network of dislocations at the interface between the membrane and the substrate. This produces a growth of a strain layer on the membrane. Under the effect of the strain energy the dislocations are supposed to change orientation to take on a corner character and thus minimise this energy. The advantage of this technique bears on the transfer of the membrane on the totality of the substrate. However, there is no guarantee of the resulting relaxation and there are doubts on the homogeneity of the resulting relaxation.
By way of example also the technique of molecular adhesion or << wafer bonding >> can be cited, as described especially in the article by D. M. Hansen et al., J. Cryst. Growth. 195, (1998) p. 144, aiming at transferring a membrane by molecular adhesion on a surface-oxidised substrate. The growth of the layer strain is then produced on the membrane. The atoms of the membrane presented at the interface can effect slight displacements for relaxing the strain layer. The principal advantage of this technique is the large size of the resulting surfaces. All the same, even if a compliance effect is observed, the relaxation is not total. The critical thicknesses of the deposited layers are increased, but it is still not possible to produce thick layers exempt from structural defects.
All these compliance plastic or elastic techniques do not exhibit the expected characteristics. Plastic or elastic deformation of the nucleus layer by the epitaxial layer is not or is only partially observed. On the other hand, the lateral dimensions of the resulting zones exempt from defects are too slight.
In the prior art, another family of known solutions relates to the paramorphic technique consisting of undertaking epitaxy of a strain membrane then of having it relax elastically to then undertake epitaxy commensurate with lattice. The aim of this technique is to successively deposit on a substrate a sacrificial layer and a strain membrane via epitaxy. A lithographic stage then selectively etches the membrane to produce the discs. Humid chemical etching is carried out to totally etch the sacrificial layer, including under the discs by sub-etching. The strain membrane relaxes elastically while it is no longer maintained. This strain membrane is then deposited on the substrate. The principal advantage of this technique comes from reprise of growth commensurate with lattice. However, the discs obtained are limited in size (a few hundred microns) and the conflicts of the initial parameters of the membrane are low (1% environ).
Another family of solutions described by D. S. Cao., J. Appl. Phys. 65, (1989) p. 2451 is the metamorphic method with use of buffer layers of fixed or gradual compositions or even super-networks. The buffer layers have a lattice parameter which is different to that of the substrate. Growth of these layers is generated for thicknesses greater than the critical thickness. The buffer layer thus relaxes via generation of dislocations and retrieves its non-strained lattice parameters. The growth of the desired active layer is thus realised on these buffer layers of lattice parameters different to those of the original substrate. The first difficulty of this technique originates from confinement of the dislocations in the buffer layer which is not total, emerging dislocations always being presented in the active layer degrading the properties of the latter. The second originates from the appearance of coarseness at the surface of the buffer layer which can degrade the expected properties of the active layer.
The prior art has also proposed via document JP 2000 0091 627 a technique for manufacturing light emitters consisting of making a deposit of a polycrystalline material with fine grains, followed by thermal treatment. This annealing allows atomic rearrangement which leads to the increase of grain size. However, this technique gives no guarantee for homogeneity of reorganisation and does not generate epitaxial growth of any layer commensurate with lattice with its substrate.
The result of analysis of the different techniques known to date is the observation that they do not give satisfaction in practice. There is the apparent need to be able to utilise a technique effecting the epitaxial growth of any layer commensurate with lattice with its substrate.